In general, a compressor-type refrigerator comprises at least a compressor, a condenser, an expansion mechanism (expansion valve, etc.), an evaporator and a drier, and a mixed liquid comprising a refrigerant and a lubricating oil is circulated in the closed system of the refrigerator. In the compressor-type refrigerator of that type, in general, dichlorodifluoromethane (R12), chlorodifluoromethane (R22) and the like have heretofore been used as refrigerants and various mineral oils and synthetic oils as lubricating oils.
However, since R12 and R22 will bring about environmental pollution, as destroying the ozone layer existing in the stratosphere, their use is being severely controlled in all the world. Given the situation, new refrigerants, non-chlorine Flon compounds such as hydrofluorocarbons have become specifically noted. Since such non-chlorine Flon compounds, for example, hydrofluorocarbons such as typically R134a will not destroy the ozone layer and can be substituted for R12 and the like without almost changing or modifying the structure of conventional refrigerators, they are favorable for refrigerants for compressor-type refrigerators.
The properties of these new Flon-substituent refrigerants are different from those of conventional Flon refrigerants; and it is known that refrigerator oils capable of being used along with these comprise a base oil component selected from, for example, polyalkylene glycols, polyesters, polyol esters, polycarbonates, polyvinyl ethers and alkylbenzenes having particular structures, and various additives added to the base oil component.
However, these refrigerator oils are seriously problematic in practical use in that, when used in the atmosphere comprising any of the above-mentioned refrigerants, their lubricity is poor and, in particular, they cause increased abrasion loss between aluminium materials and steel materials constituting compressors for air-conditioning refrigerators. Rotary-type, scroll-type and reciprocation-type compressors are used for air-conditioning refrigerators, and they have sliding members of a combination of aluminium materials and steel materials. In rotary-type compressors, for example, the bearing is the sliding member; in scroll-type compressors, the Oldham's coupling ring is the member; and in reciprocation-type compressors, the con'rod (aluminium)/piston pin (steel) member is the member. Regarding their condition for lubrication, the bearing and the Oldham's coupling ring act in an area which shall bear relatively low stress and in which the lubricating oil used exhibits its oily effect (this area is hereinafter referred to as an oil region); while the con'rod/piston pin member acts in an area which shall bear relatively high stress and which therefore requires the extreme-pressure effect of the lubricating oil used therein (this area is hereinafter referred to as an extreme-pressure region). In that situation, desired are refrigerator oils usable in any and every type of compressors, to which, therefore, desired are additives effective for reducing friction and abrasion in both regions, the oil region and extreme-pressure region.
For lubricity improvers for refrigerator oils, heretofore known are orthophosphates such as tricresyl phosphate (hereinafter referred to as TCP), triphenyl phosphate (hereinafter referred to as TPP), etc. These additives are effective for sliding members of a combination of steel materials and steel materials, but are not for those of a combination of steel materials and aluminium materials since they do not have the ability to reduce friction in the extreme-pressure region. Therefore, for ensuring good lubricity around them, the steel-aluminium sliding members require extreme-pressure agents substitutable for the conventional lubricity-improving additives.
On the other hand, another lubricity improver, sorbitan mono-oleate is proposed. This is effective for reducing friction in the oil region, but is problematic in that its volume resistivity is low.
The present invention has been made from the viewpoint as above, and its object is to provide a refrigerator oil composition of good lubricity, which is especially effective for reducing the friction in both the oil region and the extreme-pressure region in the sliding area between aluminium materials and steel materials and which is favorable to lubricating oil for refrigerators using non-chlorine Flon refrigerants such as R134a and the like that do not bring about environmental pollution.